Aluminum base alloy



Fatented New. i941- STATES PAT EN ALUMINUM BASE ALLOY Adolf Beck, Gustav Siebel, and Hugo Vosskiihler,

Bitterfeld, Germany, assignors, by mesne assignments, to Walther H. Duisberg New York,

Noni-swing. Application July 12, 1938, Serial No. 218,760. In Germany July 24, 1937 1' Claim.

This invention relates to aluminum'alloys susceptible to improvement by heat treatment and workpieces made from such alloys and aims at providing alloys having enhanced mechanical strength properties aswell as resistanceto inter (soaking") step followed by 'quenching, -and storage atroom temperature, or at moderately elevated temperatures (of the order of 100 to 150 C.), such heat-treatment resulting in an improvement of the mechanical strength properties of such alloys, The resistance to corrosion of such alloys, particularly by seawater, is how-1 ever unsatisfactory. In order to improve the resistance to corrosion of the aforesaid aluminum-niagnesium-zinc alloys it has also been proposed to increase" the magnesium content in such enisation and by the "application of suitable annealing temperatures-a certain proportion of the compound 'MgZm together-with a certain pro-. portion of the compound AlaMgz will bcome segregated in a state of imiform distribution, the segregation of the compound AlaMgs producing a substantial improvement in the corrosion resistance of the alloys, while the segregation of the MgZm compound results in the known improvement of the mechanical strength properties of the alloys (cf. British patent specification No. 438,512).

Further investigations, however, have shown that the resistance to stretch-corrosion of the alloys treated in the aforesaid manner, and also of the workpieces made therefrom is unsatisfactory. Thus'whereas, for example, a sheet of an alloy containing:

Percent Magnesium 5.5

Zinc 4.0 Manganese 0.5 Balance aluminum.

and subjected to the afoi'edescri-bedheat-treatment exhibited a tensile strength of 48 kgs. per square millimetre and an elongation of 8.5%,as delivered, and after treatment for 12 months nesium in aluminum. It speedily transpired,

the seawater spray test, in the unstressed state,

still exhibited a tensile strength of 47 kg's. per

square millimetre and an elongation of 8.5%, it-

was found that the same sheet, whenbent into a loop and thus stressed, showed signs of splitting after only 2 to 3 days exposure in the seawater spray test.

The present invention aims at improving .the

resistance to corrosion generally, and particularly at substantially reducing the sensitiveness to stretch-corrosion of the foregoing alloys, and for this purpose provides for the incorporation in the said alloys of chromium and/or calcium in mounts of from about 0.05 to about 1.0% and preferably about 0.3%.

The"incorporation of chromium and/or calcium in the aforesaid proportions not only reduces the sensitiveness to stretch-corrosion of the foregoing alloys, after having been subjected to' the aforesaid heat-treatment, but it has furthermore been found that the presence of chromium and/or calcium in the aforesaid proportion produces further beneficial eifects.

Some time ago it was suggested (cf. g. 'German specification No. 528,025) that the resistance to corrosion, particularly by seawater, of the binary aluminum magnesium alloys containing up to 10% of magnesium, may be substantially improved by subjecting such alloys to a homogenising ("soaking) treatment so as to establish a homogeneous solid solution of the maghowever, that such alloys rapidly broke down in the presence of corroding media when accidentally or deliberately heated during further working or use, to moderately elevated temperatures 0! the order of about 100 C. and it was therefore suggested (of. British patent specification No. 432,351) to subject such alloys, subsequent to said homogenising heat-treatment, to a further de liberate heat-treatment at somewhat higher temperatures so as to produce segregation of particles of the compound Ala gz in a finely dispersed form in the aluminum base matrix.

It'has now been observed that similar phenomena occur in the afore-described aluminummagnesium=-zinc alloys in which the magnesium content is substantially increased beyond the proportion necessary for the formation of the compound MgZnz, whenever such alloys contain the excess magnesium in a state of homogeneous solid solution. On the other hand, it has further been recognised that the susceptibility of the aforesaid homogenised alloys to deterioration in on accidental or deliberate heating to temperatures of the order of 100 C. is substantially removed by the presence oi! calcium and/or chromium in the aforespecifled proportions, i. e. in

amounts of from 0.05 to about 1.0% and prei'erably about 0.3%.

Since it is known that the mechanical strength properties of alloys containing the compound MgZnz in proportions exceeding its solid solubility in the aluminum base matrix, may be improved also by homogenlsing with subsequent quenching and storing at ordinary, or ageing at moderately elevated temperatures (about 100 to 150 0.), the aforesaid observations open up the possibility of subjecting also alloys, in which the magnesium content substantially exceeds that called for by the formation oi the compound MgZnz, to a similar heat-treatment without running the risk of impairing the favourable corrosion resistance properties of the latter alloys. always provided that calcium and/or chromium are present in such alloys in the aforespecifled proportions.

The present invention therefore contemplates inter alia, aluminum alloys susceptible to improvement by heat-treatment, such alloys being characterised by a content of between about 2' and about 6% of zinc, a magnesium content of up to about 12%, and a content of chromium and/or calcium of up to 1.0%, if desired with the customary addition of manganese of up to about 1.0%, the content of magnesium of the alloys exceeding that required for the formation, with the zinc present, of the compound MgZm by at least about 3.0%.

The invention further contemplates the heattreatment of workpieces consisting of the aforesaid aluminum alloys with a view to producing high mechanical strength properties combined with a high resistance to intercrystallineas well as stretch-corrosion, particularly'by seawater, by homogenising the alloys andsubsequently either storing them at room temperature or ageing them at moderately elevated temperatures below about C.

The improvements obtained by the present invention are best illustrated by the Iollowing comparative tests:

Sheets made of an alloy containing 7 Per cent Zinc M) Magnesium 6.0 Manganese 0.25

Balance aluminum An aluminum base alloy consisting of between 5.5% and 6.0% of magnesium, about 4.0% of zinc, between about 0.25% and 0.5% of manganese, and between about 0.25% and 0.3% oi chromium.

ADOLF BECK. GUSTAV SIEBEL. HUGO VOSSKfiHLER. 

